The SnAgCu solder joint integrity in WLCSP for green conversion

WLCSP with Ni/Cu UBM and SnAgCu solder interconnect comply completely with European ROHS regulation. SnAgCu solder exhibits best solder joint thermal fatigue life in organic laminate BGA type package to organic PCB during gentle TCT condition, but whether means it can also work well in silicone die level package to organic PCB due to larger CTE mismatch during temperature cycling, is still necessary to be explored. In this report, the interfacial metallurgical reactions of solder joint of the combination of 0.3 mm SnAgCu solder ball and different melting point solder pastes such as Sn37Pb and Sn3.0Ag0.5Cu will be investigated in the wafer level CSP package with 0.5mm ball pitch. After appropriate SMT process at low, medium and high reflow temperature respectively on NSMD FR4 PCB with Cu-OSP surface finish, the sample is subject to TCT (-40-125°C) 500 and 1000 cycle, then are followed by solder joint cross-section observation with SEM/EDX. The diverse failure mode on the metallurgical evolution in the interface and solder bulk itself is recorded to illustrate the solder joint integrity. The relationship among the fracture morphology and solder paste type is concluded as well. Furthermore, one new technology with polymer collar structure around the solder ball is integrated to discuss the whole SnAgCu solder joint integrity enhancement during the TCT test. A mature wafer level CSP with Sn37Pb solder ball will be used for comparison as well.